Sodar systems employ directed sound waves to detect atmospheric phenomena such as wind speed. Meteorological lidar systems use laser beams for the same purpose. The apparatuses housing the active transducers of these systems are generally deployed in fixed, carefully oriented positions in the field, such that the angles of emitted and detected signals have known relations to vertical and horizontal coordinates. Usually this requires leveling the platform upon which the transducers are mounted, and turning the platform such that it is aligned with a compass point, e.g., due north. Such systems calculate wind speeds and directions based on the prerequisite fixed orientation of the transducers.
Wind speed measurements made for the purposes of wind energy resource assessment are expected to be rather accurate; typically ±0.5 mph (approximately ±0.23 m/s). As such, the errors that would be introduced by tilting of the apparatus of only a few degrees are important to correct in this application. Additionally, at many sites wind speed can vary to an extent which significantly affects the economic viability of a potential wind turbine installation over a very short geographic distance. As a result, precise and accurate information regarding the location at which a measurement for wind energy was made is essential.
Existing and near future wind energy installations are on dry land or near-shore sea locations. Resource assessment equipment for such sites can and has been mounted on stable foundations on land and on the near-shore seabed. Resource availability and other siting issues related to these terrestrial and near-shore sites are motivating research, including resource assessment studies, into deeper water installations where fixed foundations are prohibitively expensive. Buoy, barge, or ship mounted wind measurement equipment, typically used to measure wind velocity in such locations, is inadequate for resource assessment in several respects. The measurements are typically made within a few meters of the sea surface, while resource assessment data is preferably taken at turbine hub height, typically 60-110 meters above the sea surface for modern utility scale turbines. Further, the accuracy of conventional anemometry mounted on floating structures is adversely affected by the motion of these structures. Also, the location of these sensors relative to their support structures is often such that the structures adversely affect accuracy by influencing the airflow. Finally, in particular for ship-based measurements, the duration of measurement is inadequate for resource assessment, where measurement periods of up to a year or more are considered necessary for accurate resource assessment.
To date no sodar or lidar equipment has been built which is suitable for making measurements sufficiently accurate for resource assessment on non-stationary structures.